1. Field of the Invention
The present invention relates to LED drive circuits that drive an LED (light-emitting diode) unit, and to LED light-emitting devices provided with such LED drive circuits. More particularly, the present invention relates to an LED drive circuit that drives an LED unit with an alternating-current power, and to an LED light-emitting device provided with such an LED drive circuit.
2. Description of Related Art
In recent years, as increasingly high capacities and hence increasingly high densities have been sought in recording media for recording video and audio data, short-wavelength laser light has been sought after as laser light to be shone onto an optical disc that is a type of recording medium. On the other hand, there has been developed an LED that emits blue light (a blue LED). Along with the development of the blue LED, an LED that emits white light (a white LED) has also been developed, and has come to be increasingly used in a backlight of a liquid crystal display (LCD), an illuminating device, and the like. Now that the LEDs have been available in an increasing number of colors, they are used not only in illumination but also in an ever-widening range of applications such as neon signs and various industrial applications.
Offered as an LED drive circuit for supplying a drive current to an LED unit composed of a plurality of LEDs are an LED drive circuit operating on an AC (alternating-current) power and an LED drive circuit operating on a DC (direct-current) power. An example of an LED drive circuit in which the LED unit is driven by the AC current is seen in JP-A-H11-330561 (hereinafter “Patent Document 1”) and JP-A-2002-015606 (hereinafter “Patent Document 2”). As shown in FIG. 21, in this LED drive circuit, two LED groups 100x and 100y are connected in parallel with an AC power source 101 in such a way as to point in different directions, and LEDs 1000a forming the LED group 100x are so connected as to point in one direction and LEDs 100b forming the LED group 100y are so connected as to point in the other direction, whereby the LED groups 100x and 100y are made, by turns, to emit light every half-cycle of the AC power.
An LED illuminator of Patent Document 1 is provided with a current-limiting transformer for limiting an alternating current supplied from the AC power source, whereby a current passed through the LEDs forming the LED group is limited with the impedance of the current-limiting transformer. An LED illuminating device of Patent Document 2 is provided with a plurality of load circuits, each being composed of the LED groups 100x and 100y connected in parallel in such a way as to point in different directions, as shown in FIG. 21, and each being connected in parallel with the AC power source. In addition, an inductor is connected in series to one of the load circuits, and a capacitor is connected in series to the other. Accordingly, currents having different phases are made to pass through the LED groups of each load circuit, whereby power factor is improved. Furthermore, by arranging the LED groups through which currents having different phases pass in such a way as to be adjacent to each other, optical distortion is reduced.
Alternatively, an LED drive circuit is configured as seen in JP-A-2002-231471 (hereinafter “Patent Document 3”), JP-A-2004-327152 (hereinafter “Patent Document 4”), and JP-A-2004-111104 (hereinafter “Patent Document 5”). As shown in FIG. 22, in this LED drive circuit, an AC power from an AC power source 150, which is a commercial power source, is converted into a DC power by an AC/DC converter 151, and a drive circuit 152 is supplied with the resultant DC power to drive an LED group 153. An LED lighting device of Patent Document 3 is provided with a step-up chopper circuit for stepping up a direct-current voltage from the DC power source. In this chopper circuit, based on the sum of a voltage signal commensurate with the magnitude of a current passed through the LED group and a voltage signal indicating the amount of dimming of the LED group, a duty ratio at which the switching element is operated is determined, and a direct-current voltage commensurate with the amount of dimming is applied to the LED group.
As is the case with the LED lighting device of Patent Document 3, an LED lighting device of Patent Document 4 is provided with a step-up chopper circuit for stepping up a direct-current voltage from the DC power source. Additionally, a PWM (pulse width modulation) circuit is provided that determines a duty ratio at which a switching element provided in the step-up chopper circuit is operated. An effective value of a phase-controlled alternating-current voltage from the AC power source is detected, and a duty ratio commensurate with the effective value thus detected is set, whereby dimming control of the LEDs is performed.
An LED lighting device of Patent Document 5 is provided with, as an LED group, an LED group of white LEDs and an LED group of colored LEDs, and a control circuit for performing dimming control of the white LED group and the colored LED group. The white LED group and the colored LED group are subjected to dimming control by the control circuit, whereby the brightness and emission color at the time of maximum light-emission and the brightness and emission color at the time of dimming control are continuously changed.
Furthermore, a phase-controlled dimmer provided with a phototriac coupler into which a phototriac and an LED are coupled together has been proposed (see JP-A-2006-032030 (hereinafter “Patent Document 6”), JP-A-2006-032031 (hereinafter “Patent Document 7”), JP-A-2006-032032 (hereinafter “Patent Document 8), and JP-A-2006-032033 (hereinafter “Patent Document 9”)). In LED lighting devices of Patent Documents 6 to 9, since an alternating-current voltage from the AC power source is subjected to full-wave rectification and is then supplied to the LED group, they are so configured as to prevent a malfunction that would occur near the minimum limit of dimming when phase control is performed in a similar manner every half-cycle of the alternating-current voltage.
As described above, different LED light-emitting devices provided with different LED drive circuits that drive an LED with the AC or DC power have been realized. However, the configurations of Patent Documents 1 and 2 simply aim to achieve a stable driving of LEDs and efficient power consumption, and lacks the circuit configuration for performing dimming control.
On the other hand, in the configurations of Patent Documents 3 and 4, dimming control is performed. However, these configurations not only make it necessary to make the step-up chopper circuit convert a direct-current voltage obtained as a result of AC-to-DC conversion to a value commensurate with the amount of current to be supplied to the LED group, but also make it indispensable to use an AC/DC converter when a commercial power source is used. As a result, with the configurations of Patent Documents 3 and 4, the circuit configuration increases not only in complexity but also in size.
With the configuration of Patent Document 5, although dimming control is performed on the white LED group and on the colored LED group, they are not individually subjected to different dimming control, but to the dimming control based on the same phase control. As a result, although it is possible to continuously change the brightness and emission color at the time of maximum light-emission and the brightness and emission color at the time of dimming control, it is impossible to obtain an optimum brightness and emission color by performing dimming control individually on the different LED groups.
Incidentally, also in the configuration of Patent Document 5, a direct-current voltage obtained as a result of AC-to-DC conversion is converted into a high-frequency voltage by an inverter circuit or the like, so as to make it possible to perform dimming control. As a result, like the configurations of Patent Documents 3 and 4, the circuit configuration of the drive circuit increases not only in complexity but also in size.
Since Patent Documents 6 to 9 are directed to the dimmers for use in illuminating devices having a white light as a load, the amount of current passed through the load is adjusted every half wave by performing phase adjustment every half wave in a similar manner, and an LED unit is so configured as to supply a direct-current electric power to LEDs by performing ill-wave rectification by using a diode bridge, the LEDs being arranged in series so as to point in the same forward direction. As a result, it is impossible to perform dimming control individually on a plurality of LED groups by performing different dimming control every half wave.